U.S. patent application number 14/688790 was filed with the patent office on 2015-08-13 for methods and devices for fluid enhanced microwave ablation therapy.
This patent application is currently assigned to THERMEDICAL, INC.. The applicant listed for this patent is Thermedical, Inc.. Invention is credited to Michael G. Curley.
Application Number | 20150223882 14/688790 |
Document ID | / |
Family ID | 51530905 |
Filed Date | 2015-08-13 |
United States Patent
Application |
20150223882 |
Kind Code |
A1 |
Curley; Michael G. |
August 13, 2015 |
METHODS AND DEVICES FOR FLUID ENHANCED MICROWAVE ABLATION
THERAPY
Abstract
Devices and methods for fluid enhanced ablation therapy using
microwave antennas are described herein that provide improved heat
transfer into a target volume of tissue and improved antenna
cooling compared to prior art designs. In one embodiment, fluid can
be introduced into a target volume of tissue along with the
delivery of therapeutic energy from a microwave antenna positioned
within the target volume of tissue. The fluid can become heated by
cooling the microwave antenna and, if additional heating of the
fluid is desired, a heating element disposed within the microwave
antenna can supplementally heat the fluid prior to introduction
into the target volume of tissue.
Inventors: |
Curley; Michael G.; (Weston,
MA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Thermedical, Inc. |
Waltham |
MA |
US |
|
|
Assignee: |
THERMEDICAL, INC.
Waltham
MA
|
Family ID: |
51530905 |
Appl. No.: |
14/688790 |
Filed: |
April 16, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13837295 |
Mar 15, 2013 |
9033972 |
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14688790 |
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Current U.S.
Class: |
606/33 |
Current CPC
Class: |
A61B 2018/00529
20130101; A61B 2018/00714 20130101; A61B 2018/00029 20130101; A61B
2018/00821 20130101; A61B 2018/00351 20130101; A61B 2018/00994
20130101; A61B 2018/044 20130101; A61B 2018/00702 20130101; A61B
2018/0066 20130101; A61B 2018/00642 20130101; A61B 2018/00017
20130101; A61B 2018/00898 20130101; A61B 18/1815 20130101; A61B
2018/00011 20130101; A61B 2018/00577 20130101 |
International
Class: |
A61B 18/18 20060101
A61B018/18 |
Claims
1. A microwave ablation device, comprising: an elongate body having
an inner conducting element surrounded by a dielectric insulator,
an outer conducting element coaxially disposed around the
dielectric insulator and defining an outer wall, and at least one
fluid channel extending through the elongate body parallel to a
longitudinal axis of the microwave antenna, the at least one fluid
channel including at least one opening to allow fluid to flow into
tissue surrounding the elongate body; wherein the inner and outer
conducting elements are configured to deliver microwave energy to
tissue surrounding the elongate body.
2. The device of claim 1, wherein the at least one opening is
positioned at a location proximal to a distal end of the inner
conductive element.
3. The device of claim 1, wherein the at least one opening
comprises a plurality of openings spaced along a longitudinal axis
of the at least one fluid channel.
4. The device of claim 1, wherein the at least one opening
comprises a plurality of openings spaced circumferentially around a
longitudinal axis of the at least one fluid channel.
5. The device of claim 1, wherein the at least one fluid channel
extends through the dielectric insulator between the inner
conducting element and the outer conducting element, and wherein
the at least one opening extends through the outer conducting
element.
6. The device of claim 1, wherein the microwave antenna has a
cylindrical shape, and wherein the inner conducting element,
dielectric insulator, and outer conducting element are coaxially
aligned.
7. The device of claim 1, wherein the at least one fluid channel
includes a heating element disposed therein, the heating element
being positioned proximal to the at least one opening and
configured to heat fluid flowing through the channel.
8. The device of claim 1, further comprising a shield disposed
around and coaxially aligned with the outer conducting element.
9.-20. (canceled)
Description
FIELD
[0001] The present invention relates generally to fluid enhanced
ablation therapy, and more particularly, to devices and methods for
conducting fluid enhanced ablation therapy using microwave
antennas.
BACKGROUND
[0002] Ablation therapy uses heat to kill undesirable tissue.
Energy from various sources, often radiofrequency (RF) electrical
energy, but also microwave, ultrasound, or laser energy, is used to
heat tissue to a therapeutic temperature and thereby kill the
tissue. This therapy can be used to treat various small tumors,
including malignant tumors such as liver cancer or benign tumors
such as uterine fibroids, by overheating and killing the tumorous
tissue. It can also be used to ablate arrhythmogenic heart tissue
that causes arrhythmias, such as atrial flutter. While ablation
therapy is often used to treat small amounts of tissue such as
those mentioned above, conducting the therapy for larger tumors, or
for other conditions that require treating large volumes of tissue,
remains difficult.
[0003] One prior art solution for ablating larger volumes of tissue
has been the use of microwave energy in place of RF electrical
energy. Microwave energy can be better suited for use in large
treatment volumes because its energy can penetrate farther into
tissue than RF energy. For example, microwave energy at 915 MHz can
penetrate approximately 4 cm (in theory) into tissue, as compared
to RF energy that is typically dissipated beyond about 1 cm from
the electrodes that deliver the energy. Therefore, microwave energy
can directly heat a greater volume of tissue than RF energy.
[0004] Microwave energy can have additional advantages over other
energy sources as well. For example, microwaves propagate through
all types of tissues and non-metallic materials, including charred
and desiccated tissues that can be created during the ablative
process. In contrast, when tissue becomes charred or desiccated by
RF ablation the impedance of the tissue rises dramatically, making
it difficult to pass further current through the tissue and
effectively terminating the therapy. Still further, microwaves can
deliver greater levels of direct heating energy as compared to
other ablation energies, which can be advantageous when ablation is
conducted in organs with high blood perfusion or near sources of
blood flow, such as veins or arteries, that can draw heat away from
a target tissue volume.
[0005] However, microwave energy does have drawbacks as well. For
example, while the depth of treatment may extend farther into
tissue than with RF energy, it is still limited. As mentioned
above, in theory the field is typically dissipated at about 4 cm
from the microwave antenna. In practice, the cylindrical
configuration of microwave antenna exacerbate the radial spreading
of that energy into tissue being treated, resulting in dissipation
within 2 cm of the cylindrical antenna. Treating larger volumes of
tissue therefore requires repositioning the antenna or using
multi-antenna arrays. Further, the dynamics of the microwave field
can become complex when multi-antenna arrays are used. As a result,
therapy procedure times and costs can be significantly increased
due to either repositioning a single probe several times or setting
up a multi-antenna array.
[0006] In addition, the microwave energy deposition field is static
and is defined by the electromagnetic properties of the surrounding
tissues and the geometry of the antenna itself. Within the heating
field there are volumes of tissue that are heated more or less than
others (e.g., similar to reheating food in a microwave oven). This
can lead to undesirable therapies, as some tissue can be heated to
a dangerous level by a strong microwave field (e.g., becoming
superheated and explosively converting to steam), while other
tissue can be heated to a sub-therapeutic temperature by a weaker
microwave field. Thermal energy does flow from the volumes of
tissue that are heated by strong microwave fields to those heated
by weaker microwave fields by thermal conduction, but this is a
slow and inefficient process in tissue, and does not address the
safety concern created by superheating portions of a target volume
of tissue.
[0007] Moreover, microwave energy can overheat the antenna used for
therapy application and its associated cabling used to transfer
power to the antenna from a generator. This internal heating must
be countered by limiting energy transmission or by actively cooling
the components of the microwave ablation system to prevent
undesired thermal damage to tissues in contact with the antenna or
the cabling extending to the antenna. Prior art methods for
actively cooling microwave antennas include circulating a fluid or
cryogenic gas along the length of the cable and even through the
antenna itself, but these closed-loop systems can result in large
diameter devices, e.g., due to the need for delivery and return
lumens.
[0008] Accordingly, there is a need for improved devices and
methods for conducting ablation using microwave antennas. In
particular, there is a need for microwave ablation devices and
methods that can deliver therapeutic doses of thermal energy to
large volumes of tissue and address cooling issues commonly
encountered with microwave antennas.
SUMMARY
[0009] The present invention generally provides devices and methods
for using microwave antennas in combination with fluid enhanced
ablation therapy. For example, the methods described herein
generally include placing a microwave antenna within a target
volume of tissue and simultaneously delivering microwave electrical
energy and a fluid into the tissue. The fluid introduced into the
tissue can aid in distributing the heat from the microwave field
throughout the tissue volume via convection. Furthermore, in some
embodiments, the fluid itself can be heated to a therapeutic level
prior to being introduced into the tissue. Accordingly, the fluid
can act as a therapeutic agent and the energy delivered through the
microwave antenna can replenish heat lost from the fluid into the
target tissue volume.
[0010] In addition to utilizing the fluid as a therapeutic agent to
ablate larger volumes of tissue, the devices and methods described
herein can also utilize the fluid to aid in cooling the microwave
antenna or cabling extending between a power generator and the
microwave antenna. Indeed, in some embodiments, excess heat from
the microwave antenna and/or its associated cabling can be used to
at least partially heat the fluid to a therapeutic temperature
before introducing the fluid into the target volume of tissue,
thereby putting traditionally wasted heat to use in the therapy.
Still further, in embodiments in which a cryogenic gas is utilized
to cool the antenna and/or associated cabling, a heating element
disposed within a fluid passageway can be used in combination with
the cooling action of the cryogenic gas to deliver fluid at any
temperature (e.g., any temperature above freezing) into the target
volume of tissue.
[0011] In one aspect, a microwave ablation device is provided that
includes an elongate body having an inner conducting element
surrounded by a dielectric insulator, an outer conducting element
coaxially disposed around the dielectric insulator and defining an
outer wall, and at least one fluid channel extending through the
elongate body parallel to a longitudinal axis of the microwave
antenna. The at least one fluid channel can include at least one
opening to allow fluid to flow into tissue surrounding the elongate
body, and the inner and outer conducting elements can be configured
to deliver microwave energy to tissue surrounding the elongate
body.
[0012] The devices and methods described herein can include a
variety of additional features or modifications, all of which are
considered within the scope of the present invention. For example,
in some embodiments the microwave antenna can have a cylindrical
shape, and the inner conducting element, dielectric insulator, and
outer conducting element can be coaxially aligned. In other
embodiments, the device can further include a shield disposed
around and coaxially aligned with the outer conducting element.
[0013] In other embodiments, the at least one opening can be
positioned at a location proximal to a distal end of the inner
conductive element. The at least one opening can, in some
embodiments, include a plurality of openings. The plurality of
openings can be spaced circumferentially around a longitudinal axis
of the at least one fluid channel, and/or they can be spaced along
a longitudinal axis of the at least one fluid channel.
[0014] The at least one fluid channel can, in some embodiments,
extend through the dielectric insulator between the inner
conducting element and the outer conducting element, and the at
least one opening can extend through the outer conducting element.
Furthermore, in certain embodiments, the at least one fluid channel
can include a heating element disposed therein. The heating element
can be positioned proximal to the at least one opening and
configured to heat fluid flowing through the channel.
[0015] In another aspect, an ablation device is provided that
includes at least one elongate body having proximal and distal ends
and at least one microwave antenna extending from a distal end of
the at least one elongate body. The at least one microwave antenna
can have an arced shape that defines a plane, and the device can
further include a fluid channel extending through one of the
body(s) and configured to deliver fluid into tissue from a center
point of the arc defined by the microwave antenna. The fluid
channel can also include a heating element disposed therein and
configured to heat fluid flowing through the channel.
[0016] In some embodiments, the device includes first and second
elongate bodies and first and second microwave antennas, with the
first microwave antenna extending from the distal end of the first
elongate body and the second microwave antenna extending from the
distal end of the second elongate body. The planes of the arc
shaped antennas can be angularly offset from one another such that
the first and second microwave antennas generally define a
sphere.
[0017] In another aspect, an ablation device is provided that
includes a microwave antenna having a hollow outer conducting
element, a coaxial inner conducting element extending through the
outer conducting element, a fluid channel positioned between the
outer and inner conducting elements, an inner lumen extending
through the inner conducting element, and at least one outlet port
formed in the outer conducting element and in fluid communication
with the fluid channel. The at least one outlet port can be
configured to deliver fluid flowing through the fluid channel into
tissue surrounding the microwave antenna. The inner lumen can be
configured to receive a cryogenic gas source. The fluid channel can
also include at least one heating element disposed therein and
configured to heat fluid flowing through the channel.
[0018] In still another aspect, a method for ablating tissue is
provided that includes delivering therapeutic energy through a
microwave antenna configured to be positioned within a volume of
tissue, and simultaneously delivering heated fluid into the volume
of tissue through at least one outlet port formed in the microwave
antenna, wherein the fluid is heated by contact with the microwave
antenna.
[0019] In some embodiments, heating the fluid by contact with the
microwave antenna can have the additional benefit of cooling the
antenna to prevent overheating. As a result, heat that is
traditionally wasted can be utilized in the therapy. If the fluid
is not heated sufficiently by contacting the microwave antenna, the
method can further include raising the temperature of the heated
fluid prior to introduction into the volume of tissue using at
least one heating element disposed within the microwave antenna. In
certain exemplary embodiments, the heated fluid has a heat capacity
that is equal to or greater than 2 J/ml-.degree. C. (about 1/2 the
heat capacity of tissue).
[0020] In another aspect, a method for ablating tissue is provided
that includes delivering therapeutic energy into a volume of tissue
using a microwave antenna, and cooling the microwave antenna by
delivering a cryogenic gas through at least one gas channel formed
in the microwave antenna. The method also includes delivering a
fluid into the volume of tissue through at least one fluid channel
formed in the microwave antenna, and controlling the temperature of
the fluid delivered into the volume of tissue using the cooling of
the cryogenic gas and a heating element disposed within the at
least one fluid channel. By balancing the cooling effects of the
cryogenic gas with the output of the heating element, fluid of
almost any desired temperature can be introduced into the volume of
tissue. In certain embodiments, for example, the temperature of the
fluid delivered into the volume of tissue can be controlled to be
between a freezing temperature of the fluid and a temperature of
the volume of tissue.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] The aspects and embodiments of the invention described above
will be more fully understood from the following detailed
description taken in conjunction with the accompanying drawings, in
which:
[0022] FIG. 1 is a cross-sectional diagram of a prior art microwave
antenna;
[0023] FIG. 2 is a cross-sectional diagram of one embodiment of a
microwave antenna;
[0024] FIG. 3 is a graph showing microwave field strength and
tissue temperature when saline flow is not present;
[0025] FIG. 4 is a graph showing microwave field strength and
tissue temperature when saline flow is present;
[0026] FIG. 5 is a diagram of one embodiment of a fluid enhanced
ablation therapy system that includes a microwave antenna;
[0027] FIG. 6 is a diagram of an alternative embodiment of a
microwave antenna;
[0028] FIG. 7 is a diagram of another embodiment of a microwave
antenna;
[0029] FIG. 8 is a top view of the microwave antenna of FIG. 7;
[0030] FIG. 9 is a cross-sectional diagram of an alternative
embodiment of a microwave antenna;
[0031] FIG. 10 is a cross-sectional diagram of yet another
embodiment of a microwave antenna;
[0032] FIG. 11 is a cross-sectional diagram of one embodiment of a
microwave antenna assembly;
[0033] FIG. 12 is a cross-sectional diagram of the microwave
antenna assembly of FIG. 11 at location A-A; and
[0034] FIG. 13 is a cross-sectional diagram of the microwave
antenna assembly of FIG. 11 at location B-B.
DETAILED DESCRIPTION
[0035] Certain exemplary embodiments will now be described to
provide an overall understanding of the principles of the devices
and methods disclosed herein. One or more examples of these
embodiments are illustrated in the accompanying drawings. Those
skilled in the art will understand that the devices and methods
specifically described herein and illustrated in the accompanying
drawings are non-limiting exemplary embodiments and that the scope
of the present invention is defined solely by the claims. The
features illustrated or described in connection with one exemplary
embodiment may be combined with the features of other embodiments.
Such modifications and variations are intended to be included
within the scope of the present invention.
[0036] The present invention is generally directed to devices and
methods for fluid enhanced ablation therapy using microwave
antennas. As mentioned above, the devices and methods described
herein generally include placing a microwave antenna within a
target volume of tissue and simultaneously delivering microwave
electrical energy and a fluid into the tissue. The fluid introduced
into the tissue can aid in distributing the heat from the microwave
field throughout the tissue volume via convection. Furthermore, in
some embodiments, the fluid itself can be heated to a therapeutic
level prior to being introduced into the tissue. Accordingly, the
fluid can act as a therapeutic agent and the energy delivered
through the microwave antenna can replenish heat lost from the
fluid into the target tissue volume. The introduction of fluid into
the volume of tissue in combination with the microwave field can
effectively destroy tissue far beyond the approximately 2 cm
practical limit of conventional microwave antennas, and even well
beyond the 4 cm theoretical limit of conventional microwave
antennas mentioned above. Further, the devices and methods
described herein can allow for treating large volumes of tissue
with a single probe in a single position, thereby reducing therapy
time and cost. Of course, the devices and methods described herein
can also be applied to microwave antenna architectures that utilize
multiple probes.
[0037] Moreover, the devices and methods described herein can
provide enhanced cooling to a microwave antenna or cabling
extending between a power generator and the microwave antenna.
Further, the excess heat drawn from the microwave antenna can raise
the temperature of the fluid prior to its introduction into the
target volume of tissue, thereby putting heat that is typically
wasted to use in the therapy. The heat from the microwave antenna
alone can be used to heat the fluid, or the fluid can be
supplementally heated by a heating element prior to being
introduced into the target volume of tissue. Still further, in
embodiments in which a cryogenic gas is used to cool the microwave
antenna and/or its cabling, a balance of the cooling effects of the
cryogenic gas and the output power of a heating element can be used
to provide fluid at any desired temperature--even temperatures
below room or body temperature.
[0038] FIG. 1 illustrates one embodiment of a prior art microwave
antenna 100 and its associated microwave field shape and strength
at various distances from the antenna. The antenna 100 is a
triaxial architecture in a 17 gauge probe, as described by Brace, C
L Crit Rev Biomed Eng. 2010; 38(1): 65-78. The triaxial antenna 100
is a generally cylindrical antenna having a central inner conductor
102, such as an active electrode, extending along a longitudinal
axis 103 thereof. The inner conductor 102 can be surrounded by a
dielectric insulator 104 that is coaxially aligned with, and
extends over an outer surface of, the inner conductor. Extending
over an outer surface of the dielectric insulator 104 can be an
outer conductor 106, such as a return electrode. In addition, a
shield 108 can extend over an outer surface of the outer conductor
106.
[0039] The various components of the microwave antenna 100 can
extend for different lengths along the longitudinal axis 103 of the
antenna. For example, the inner conductor 102 and the dielectric
insulator 104 can extend along the entire length of the antenna
100, and the inner conductor 102 can be exposed at a distal end 110
thereof. The outer conductor 106 can extend to a first endpoint 112
that is proximal to the distal end 110, and the shield 108 can
extend to a second endpoint 114 that is proximal to the first
endpoint 112. This arrangement leaves specific sidewall portions of
the inner and outer conductors 104, 106 exposed and results in a
tear-drop shaped microwave field formed around the antenna 100, as
shown by field-strength lines 116, 118, 120.
[0040] As can be seen in the figure, the microwave field strength
is strongest (e.g., 800 V/m) along the field strength line 116 that
is closest to the antenna 100. The field strength dissipates with
distance from the antenna 100, as shown by field strength lines 118
(e.g., 600 V/m) and 120 (e.g., 200 V/m). In particular, the field
strength can be greatest within about 4 mm of the antenna 100, and
can fall to negligible levels by about 1 cm from the antenna.
[0041] Thermal conduction can serve to treat volumes of tissue
extending farther than 1 cm from the antenna 100, as heat can flow
from the tissue heated directly by the microwave field into tissue
that is unheated due to the thermal gradients that result from the
non-uniform field. As mentioned above, however, it is accepted that
the treatment field from a single antenna 100 is limited to a
distance of about 4 cm, and more practically about 2 cm, from the
antenna. Treating larger volumes of tissue can require the use of
additional microwave antennas, or the subsequent repositioning of
the antenna 100 in various portions of the larger volume to be
treated. Using additional antennas can be challenging and often
adds significant time to the therapy because the desired relative
placements of the multiple antennae can be difficult to achieve and
because the resulting microwave field can be difficult to
monitor.
[0042] The devices and methods of the present invention can utilize
fluid enhanced ablation therapy to address these challenges. Fluid
enhanced ablation therapy, as mentioned above, is defined by
passing a fluid into tissue while delivering therapeutic energy
from an ablation element. The delivery of therapeutic energy into
tissue causes hyperthermia in the tissue, ultimately resulting in
necrosis. A variety of fluids can be used to conduct the therapy.
In some embodiments, a sterile normal saline solution (defined as a
salt-containing solution) can be utilized. However, other liquids
may be used, including sterile water, Ringer's solution, or
concentrated saline solution. A fluid can be selected to provide
the desired therapeutic and physical properties when applied to the
target tissue and a sterile fluid is recommended to guard against
infection of the tissue. In an exemplary embodiment, the fluid has
a heat capacity that is substantially the same as or greater than
the heat capacity of tissue such that, as the fluid is delivery to
the target tissue, the fluid can retain its temperature as heat is
exchanged with the tissue. For example, the fluid can have a heat
capacity of greater than about 4 J/ml-.degree. C., and more
preferably greater than about 2 J/ml-.degree. C. (which is about
1/2 the heat capacity of tissue).
[0043] One example of fluid enhanced ablation therapy is the
SERF.TM. ablation technique (Saline Enhanced Radio Frequency.TM.
ablation) described in U.S. Pat. No. 6,328,735, which is hereby
incorporated by reference in its entirety. The SERF ablation
technique delivers fluid heated to a therapeutic temperature into
tissue along with RF energy. Delivering heated fluid in combination
with microwave power can enhance the ablation treatment because the
fluid flow through the extracellular space of the treatment tissue
can increase the heat transfer through the tissue significantly.
The flowing heated fluid can convect thermal energy from the
antenna farther into the target tissue. In addition, the fact that
the fluid is heated to a therapeutic temperature increases the
amount of energy that can be imparted into the tissue.
[0044] An advantage of fluid enhanced ablation therapy is that the
use of flowing saline can avoid overheating the tissue located
adjacent to the microwave antenna because the heat cannot be
efficiently transported away from the antenna. During fluid
enhanced ablation therapy, the therapeutically heated fluid can
convect heat deeper into the target tissue, thereby reducing tissue
charring near to the antenna. Further, because the fluid is heated
to a therapeutic level, it does not act as a heat sink that draws
down the temperature of the surrounding tissue. Therefore, the
total volume of tissue that can be heated to therapeutic
temperatures is increased. For example, experimental testing using
RF electrical energy has demonstrated that a volume of tissue
having a diameter of approximately 8 cm (i.e., a spherical volume
of approximately 156 cm.sup.3) can be treated in 5 minutes using
fluid enhanced ablation therapy techniques.
[0045] In addition, fluid enhanced ablation therapy devices have a
greater number of parameters that can be varied to adjust the shape
of the treatment profile according to the tissue being treated. For
example, operators or control systems can modify parameters such as
saline temperature (e.g., from about 40.degree. C. to about
80.degree. C.), saline flow rate (e.g., from about 0 ml/min to
about 20 ml/min), output power (e.g., from about 0 W to about 200
W), and duration of treatment (e.g., from about 0 min to about 10
min) to adjust the temperature profile within the target volume of
tissue.
[0046] FIG. 2 illustrates one embodiment of a microwave antenna 200
that can be used in fluid enhanced ablation therapy. The antenna
200 shares the triaxial architecture of the antenna 100 discussed
above, including an inner conductor 202, dielectric insulator 204,
outer conductor 206, and shield 208 extending along a length
thereof. The antenna can have a variety of sizes according to its
intended use (e.g., laparoscopic treatment of the liver,
catheter-based treatment of the heart, etc.), the size of the
target tissue volume, etc. In one embodiment, the microwave antenna
can be mounted on a 17 gauge probe having an outer diameter of
approximately 1.5 mm and a length of about 20 cm.
[0047] To facilitate its use in fluid enhanced ablation therapy
techniques, the antenna 200 can include at least one fluid channel
formed therein to deliver fluid into tissue surrounding the
antenna. In the illustrated embodiment, the antenna 200 includes a
single tubular or annular fluid channel 210 that extends along a
longitudinal axis 203 of the antenna 200. As shown, the fluid
channel 210 can be formed in the dielectric insulator 204 and
coaxially disposed around the inner conductor 202, with the
dielectric insulator 204 separating the inner conductor 202 from
the fluid channel 210. In other embodiments, the antenna 200 can
include two separate channels that extend along opposite sides of
the longitudinal axis 203 of the antenna 200, or any number of
channels distributed around the circumference of the antenna 200.
In addition, while the illustrated fluid channel 210 is formed
through a portion of the dielectric insulator 204, in some
embodiments the at least one fluid channel can be formed at another
location (e.g., between the outer conductor 206 and shield 208, or
on an outer surface of the antenna 200).
[0048] The at least one fluid channel can include at least one
opening 212 to allow fluid to flow from within the channel into the
tissue surrounding the antenna 200. The at least one opening 212
can extend through the outer edges of the antenna 200 to provide
fluid communication between the at least one fluid channel and the
tissue surrounding the antenna 200. The at least one opening 212
can be formed at any point along the length of the antenna 200, and
the at least one fluid channel can similarly extend to any point
along the length of the antenna 200. In some embodiments, for
example, the at least one opening 212 can include a plurality of
openings formed along a distal portion of the fluid channel 210
that terminates near the mid-point of the antenna 200, thereby
introducing saline near a center of the microwave energy deposition
pattern shown in FIG. 1. The plurality of openings can be formed in
a variety of patterns, including, for example, in a pattern
extending radially around the outer circumference of the antenna
200 or extending in symmetrically opposed lines along the
longitudinal axis 203 of the antenna 200.
[0049] The at least one opening 212 can be formed in a variety of
sizes, numbers, and pattern configurations. In addition, the at
least one opening can be configured to direct fluid in a variety of
directions with respect to the antenna 200. These can include the
normal orientation (i.e., perpendicular to an antenna outer
surface), as well as orientations directed proximally and distally
along the longitudinal axis 203 of the antenna 200, including
various orientations that develop a circular or spiral flow of
liquid around the antenna. Still further, in some embodiments, the
at least one fluid channel can extend to a distal end of the
antenna 200 and the at least one opening 212 can be located on a
distal surface of the antenna. A number of manufacturing methods
are available to create the at least one opening, including, in one
embodiment, creating one or more openings having a diameter of
about 0.4 mm along a distal portion of fluid channel 210 using
laser drilling.
[0050] In addition, the at least one fluid channel 210 can include
a heating element disposed therein and configured to heat fluid
flowing therethrough. Exemplary heating elements include, for
example, assemblies that are configured to pass electrical energy
through fluid within the channel between two or more electrodes.
Exemplary designs for heating elements suitable for use with the
microwave antenna 200 and other similar devices are described in
U.S. Pat. Pub. No. 2012/0265190, which is hereby incorporated by
reference in its entirety.
[0051] One skilled in the art will appreciate that the at least one
fluid channel 210 and the at least one opening 212 can also be
included in a number of other microwave antenna architectures in
addition to the triaxial antenna 200. These can include, for
example, slot, monopole, dipole, and choked microwave antenna
architectures known in the art.
[0052] In use, fluid can be delivered into the tissue surrounding
the antenna simultaneously with microwave energy from the antenna
200. The at least one opening 212 can be positioned such that the
fluid is introduced into the tissue surrounding the antenna near
the center of maximum heating and can flow through the
extracellular space of the tissue to deliver a therapeutic dose of
thermal energy. The fluid can carry significantly more energy
deeper into the tissue because it carries the heat via
convection.
[0053] By way of example, heat flowing in a given direction due to
conduction is proportional to the thermal conductivity of the
material k times the maximum temperature T.sub.max divided by the
length scale a. By comparison, the heat carried by a flow of fluid
Q in the same direction is proportional to the heat capacity of the
fluid (e.g., saline) .rho..sub.fc.sub.f times the maximum
temperature T.sub.max times the flow velocity of the fluid
Q/4.pi.a.sup.2. Therefore, the ratio of the amount of energy
carried by convection to that carried by conduction can be
expressed by the following equation:
Convection Conduction .varies. .rho. f c f Q 4 .pi. ka ( 1 )
##EQU00001##
This proportion calculates to about 11 for typical values of tissue
thermal properties and for a flow rate of saline of about 10 ml/min
and a microwave field that peaks at about 1 cm from the antenna.
Thus, heat transfer into tissue is improved by an order of
magnitude when saline is injected into tissue along with microwave
energy.
[0054] FIG. 3 illustrates an exemplary microwave power deposition
profile 302 and tissue temperature profile 304 that can result from
the use of the microwave antenna 200 without any fluid injection
(i.e., similar to using the prior art antenna 100). While
conduction does moderate the peak temperature from what would occur
if the tissue were perfectly insulating (i.e., zero thermal
conduction), the peak temperature remains near the location of peak
power deposition, i.e., close to the antenna 200. Furthermore, both
the power deposition level and tissue temperature fall off quickly
with distance from the antenna.
[0055] In contrast, FIG. 4 illustrates an exemplary power
deposition profile 402 and tissue temperature profile 404 that can
result when the antenna 200 is used in combination with a 10 ml/min
saline flow rate. As the figure shows, the temperature profile 404
is greatly modified. In particular, the peak temperature is reduced
and the temperature profile becomes more uniform, resulting in a
more uniform thermal dose being delivered across the treatment
zone. Furthermore, the therapeutic temperatures (e.g., temperatures
sufficiently high to destroy tissue) extend deeper into the tissue,
resulting in more tissue being treated using a single antenna.
[0056] FIG. 5 illustrates one embodiment of a fluid enhanced
ablation therapy system 500 that includes a microwave antenna 502
similar to the antenna 200 described above. The antenna 502 can
have a variety of sizes according to the geometry of the target
tissue, intended method for introduction into the target tissue,
intended power output, etc. The antenna 502 can be configured for
insertion into a target volume of tissue in a variety of manners,
including, for example, laparoscopic percutaneous introduction or
catheterization via a patient's circulatory system. In one
embodiment, for example, the antenna 502 can be a probe between
about 16- and 18-gauge (i.e., an outer diameter of about 1.27 mm to
about 1.65 mm), and it can have a length that is approximately 20
cm. The antenna 502 can include a blunt distal end in some
embodiments, or it can include a pointed distal tip configured to
puncture tissue to facilitate introduction of the antenna 502 into
a target volume of tissue.
[0057] The at least one fluid channel, such as the illustrated
fluid channels 504, 506, can be coupled to a fluid reservoir 508 by
at least one fluid conduit 510. The fluid conduit 510 can be, for
example, a length of flexible plastic tubing. The fluid conduit 510
can also be a rigid tube, or a combination of rigid and flexible
tubing. The fluid reservoir 508 can have a variety of geometries
and sizes. In one embodiment, the fluid reservoir 508 can be a
cylindrical container similar to a syringe barrel that can be used
with a linear pump, as described below.
[0058] Fluid can be urged from the fluid reservoir 508 into the
fluid channels 504, 506 of the antenna 502 by a pump 512. In one
embodiment, the pump 512 can be a syringe-type pump that produces a
fixed volume flow via linear advancement of a plunger (not shown).
In other embodiments, however, other types of pumps, such as a
diaphragm pump, may also be employed.
[0059] The pump 512, as well as any other components of the system,
can be controlled by a controller 514. The controller 514 can
include a power supply and can be configured to deliver electrical
control signals to the pump 512 to cause the pump to produce a
desired flow rate of fluid. The controller 514 can be connected to
the pump 512 via an electrical connection 516. The controller 514
can also be electrically coupled to the antenna 502 using any known
electrical connections of any desired length, though typically this
connection is formed using a coaxial cable having an inner
conductor and an outer conductor to mimic, for example, the
structure of the antenna 200. Regardless of the physical form of
the connection, an inner emitting conductor 518 of the antenna 502
can be electrically coupled to the power supply and controller 514
using an electrical connection 520. Similarly, an outer return
conductor 522 can be coupled to the power supply and controller 514
using an electrical connection 524. In addition, the controller 118
can be connected to any heating assemblies disposed within the
fluid channels 504, 506 (not shown) through a similar electrical
connection.
[0060] In operation, the controller 514 can drive the delivery of
fluid into target tissue at a desired flow rate, the heating of the
fluid to a desired therapeutic temperature, and the delivery of
microwave ablative energy via the antenna 502. To do so, the
controller 514 can itself comprise a number of components for
generating, regulating, and delivering required electrical control
and therapeutic energy signals. In addition to the power supply
mentioned above, the controller 514 can include one or more digital
data processors and associated storage memories that can be
configured to perform a variety of functions, or control discrete
circuit elements that perform a given function. These functions can
include, for example, the generation of one or more electrical
signals of various frequencies and amplitudes. Furthermore, the
controller 514 can be configured to amplify any of these signals
using one or more power amplifiers. These amplified signals can be
delivered to the antenna 502 via one or more electrical connections
520, passed through tissue surrounding the antenna 502, and
returned to the power supply and controller 514 via the electrical
connections 524. The controller 514 can also include a number of
other components, such as a directional coupler to feed a portion
of the one or more microwave signals to, for example, a power
monitor to permit adjustment of the microwave signal power to a
desired treatment level. Still further, the controller 514 can
include a user interface to allow an operator to interact with the
controller and set desired therapy operating parameters or receive
feedback from the controller (e.g., warnings, indications,
etc.).
[0061] As mentioned above, the triaxial antenna 200 is one of a
variety of microwave antenna architectures known in the art. FIG. 6
illustrates an embodiment of a device 600 configured for use with a
fluid enhanced ablation therapy system that utilizes a different
embodiment of a microwave antenna. The device 600 can include an
elongate body 602 having an inner lumen extending therethrough that
is in communication with tissue surrounding the elongate body 602
via an opening 604 formed on a distal portion of the elongate body.
An arced-shape microwave antenna 606 can be housed within the inner
lumen and it can be configured to axially translate to extend
through the opening 604 into the tissue surrounding the elongate
body 602, as shown in the figure. The arced-shape antenna 606 can
define a plane, i.e., it can trace a portion of a circle or other
arc that lies within a plane (e.g., the plane of the page in FIG.
6). The arced-shape configuration of the antenna 606 can produce a
roughly spherical microwave energy deposition field. An exemplary
arced-shape antenna was formerly sold under the trade name VIVARING
by Vivant/Valleylab.
[0062] Also housed within the inner lumen of the elongate body 602
can be a fluid passageway 608 having an opening formed at or near a
distal end 609 thereof. Similar to the antenna 606, the fluid
passageway 608 can be housed within the inner lumen of the elongate
body 602 during insertion into a volume of tissue, and it can be
configured to axially translate to extend from the opening 604 such
that the distal end 609 is positioned at or near a center point of
the arced-shape antenna 606, as shown in FIG. 6.
[0063] In use, fluid can be injected into the tissue surrounding
the device 600 from the opening formed near the distal end 609 of
the fluid passageway 608 at the same time that microwave energy is
delivered into the tissue from the arc-shape antenna 606. The
introduction of fluid at or near the center point of the
arced-shape antenna is similar to the introduction of saline from
the at least one opening 212 shown in FIG. 2. That is, saline or
any other suitable fluid injected at this location acts as a point
source, and the saline flows radially in a spherical pattern from
the distal end of the fluid passageway 608. Such a source of saline
can help the therapy temperature profile from microwave energy
deposition grow in a spherical pattern.
[0064] The device 600 can be used alone, as shown in FIG. 6, or it
can be used in combination with one or more other devices to create
a more spherical volume of therapeutically treated tissue. FIG. 7
illustrates one embodiment of an assembly that uses multiple
arced-shape antennas to create such a pattern. In particular, three
elongate bodies 702, 704, 706 can be positioned within a target
volume of tissue such that their arced-shape microwave antennas
708, 710, 712 define planes that are angularly offset from one
another and that intersect one another. In the illustrated
embodiment, for example, the planes defined by the three microwave
antennas 708, 710, 712 are each offset from one another by
120.degree. such that they define three great circles (i.e., a
circle on the surface of a sphere that lies in a plane passing
through the sphere's center) of the spherical volume of tissue
disposed between the antennas. In other embodiments, however, the
antennas can be angularly offset by other amounts, including, for
example, 90.degree. (i.e., perpendicular orientation). Regardless,
a fourth elongate body 714 can be introduced parallel to the
elongate bodies 702, 704, 706 to introduce fluid into the target
tissue volume. The elongate body 714 can be positioned vertically
such that the fluid is introduced at or near the center of the
sphere defined by the arced-shape microwave antennas 708, 710,
712.
[0065] FIG. 8 shows a top view of the elongate bodies 702, 704,
706, 714, along with the arced-shape antennas 708, 710, 712. From
this view, the angular offset of the planes defined by the antennas
708, 710, 712 and the central location of the elongate shaft 714
injecting fluid into the volume of tissue are visible. In such an
embodiment, the flow of fluid from the elongate body 714 can be
perpendicular to the energy deposition pattern and can moderate the
peak temperatures, make the temperature in the therapy zone more
uniform, and extend the heating field deeper into the tissue than
is possible by using the antennas 708, 710, 712 alone.
[0066] Another alternative embodiment of a microwave antenna for
use in fluid enhanced ablation therapy is shown in FIG. 9. The
antenna 900 can include an inner conductor 912, a dielectric
insulator 914, an outer conductor 916, and a shield 918 in a
similar arrangement as antenna 200. In this embodiment, the
microwave antenna 900 can also include dual fluid channels 904, 906
symmetrically disposed on opposing sides of a longitudinal axis 908
of the antenna. In an alternative embodiment, a single annular
channel can be provided extending around the circumference of the
antenna 900. Each fluid channel 904, 906 can include a plurality of
linearly arranged openings 910 extending along the longitudinal
axis 908 of the microwave antenna 900. In such an embodiment, the
point saline source of FIG. 2 is extended by including the longer
array of openings 910. As a result, this design can inject saline
into the tissue as a line source, and the resultant saline flow
within the target volume of tissue can be radially outward and
cylindrically symmetric, rather than spherically symmetric as in
the embodiment of FIG. 2. The resulting thermal field can be a
substantially uniform temperature field extending cylindrically
around the antenna 900.
[0067] Regardless of the antenna architecture used, in some
embodiments, high fluid flow rates can provide too much cooling to
the therapy. For example, the saline flow rate needed to
effectively carry the heat delivered into the tissue can be
significant, e.g., as high as 50 ml/min or more. Saline flow into
the tissue at such rates can substantially quench the thermal
therapy, keeping the tissue below a therapeutic temperature. To
combat this problem, or to impart additional energy into the tissue
even at lower fluid flow rates, the saline or other suitable fluid
can be heated to a therapeutic temperature prior to being
introduced into the target volume of tissue.
[0068] FIG. 10 illustrates one embodiment of a microwave antenna
1000 that is similar to the triaxial fluid enhanced ablation
antenna 900 described above. It similarly includes an inner
conductor 1020, dielectric insulator 1022, outer conductor 1024,
and shield 1026 arranged in the same configuration. The antenna can
also include at least one fluid channel, such as dual fluid
channels 1002, 1004. However, each of the fluid channels in this
exemplary embodiment can further include at least one heating
element disposed therein. In the illustrated embodiment, for
example, the heating elements 1006, 1008 are disposed in the two
fluid channels 1002, 1004, respectively. The heating elements can
be used to heat fluid flowing through the fluid channels 1002, 1004
prior to their introduction into tissue surrounding the antenna
1000 via a plurality of openings 1010 formed in each of the fluid
channels.
[0069] The heating elements 1006, 1008 can have a variety of forms
but, in some embodiments, can be dual-wire electrical heating
assemblies that pass RF electrical energy between two wires 1012 to
heat the fluid. The wires can be insulated along a proximal portion
thereof extending from a power source, as described above. The
wires 1012 can be exposed along a distal portion thereof to allow
the passage of energy therebetween, and electrical shorts can be
prevented by non-conducting spacers 1014 that hold the wires 1012
apart from one another. A thermocouple 1016, 1018 or other
temperature monitoring element can also be disposed within the
fluid channels 1002, 1004 to aid an operator or control system in
regulating the power applied to the heating elements 1006, 1008 to
achieve the desired temperature of fluid prior to introduction into
tissue. Exemplary embodiments of the dual-wire heating elements
1006, 1008 are described in U.S. Pat. Pub. No. 2012/0265190, which
is incorporated by reference above.
[0070] In addition to the dual-wire heating elements described
above, alternative forms of heating elements can also be suitable
for use with the devices and methods described herein. For example,
instead of using two separated wires, a heating element can utilize
any two or more metal electrodes in contact with the saline or
other suitable fluid that can pass electrical current, thereby
heating the saline resistively. Another embodiment described in
U.S. Pat. Pub. No. 2012/0265190, incorporated by reference above,
utilizes a single wire disposed within a fluid channel formed of an
electrically conductive material. As a result, the fluid channel
itself acts as the second electrode in combination with the single
wire disposed within the channel. Similar to the dual-wire heating
element described above, a non-conducting spacer element can be
utilized to prevent electrical shorts between the single wire and
the conductive walls of the fluid channel.
[0071] In still other embodiments, a heating element can be a
high-frequency microwave antenna whose frequency is selected such
that the energy field it generates is fully dissipated within the
fluid channel. Similarly, a heating element can employ an
electrically resistive material in contact with the saline that is
heated via electrical power such that the resulting dissipated
energy can be passed into the saline through thermal
conduction.
[0072] In certain microwave antenna architectures known in the art,
a cryogenic gas has been used to control internal heating of a
proximal portion of a microwave antenna and the cabling connecting
the antenna to a power source. One such embodiment of a
cryogenically cooled microwave ablation system is described in U.S.
Pat. No. 7,244,254, which is hereby incorporated by reference in
its entirety. As described in that reference, heating of a cable
and the proximal portion of a microwave antenna coupled thereto can
be controlled by the use of a cooled gas, ideally a cryogenic gas
that can exceptionally cool when expanded due to the Joule-Thompson
effect. In use, a cryogenic gas is introduced into an internal gas
feed line disposed along the length of the power supply cable. The
gas travels through the feed line toward the proximal end of the
antenna and is allowed to expand as it reaches the antenna. The
expansion of the gas near the proximal end of the antenna can
effectively cool that portion of the antenna, and the gas can
subsequently be routed to a vent along a proximal portion of the
cable.
[0073] This design can provide adequate cooling of the cable and
antenna, but does not remedy the challenges encountered with
conventional microwave ablation related to treating larger volumes
of tissue, as explained above. FIGS. 11-13 illustrate one
embodiment of a cryogenically cooled microwave antenna assembly
configured for use with fluid enhanced ablation therapy.
[0074] With reference to FIG. 11, a microwave antenna assembly 1100
is shown that includes a microwave power cable 1102 and an antenna
1104. The illustrated antenna 1104 can be a monopole or dipole
antenna, but the principles of the invention can be applied to
other microwave antenna architectures as well. As shown in the
figure, an outer return conductor 1106 can extend along an outer
surface of the antenna assembly 1100 from the antenna 1104 to the
cable 1102 and ultimately to a power supply, such as the power
supply and controller 514 discussed above. The outer return
conductor 1106 can define an inner lumen 1107, and an inner
emitting conductor 1108 can be disposed within the inner lumen 1107
in a proximal portion of the cable 1102. A similar inner emitting
conductor 1110 can be disposed in a distal portion of the antenna
1110. The inner conductors 1108, 1110 can be coupled by an inner
gas feed line 1112 that extends along the length of a distal
portion of the cable 1102 and a proximal portion of the antenna
1104. The gas feed line 1112 can be formed from a metal tube so
that it can effectively conduct microwave power from inner emitting
conductor 1108 to inner emitting conductor 1110.
[0075] A proximal portion of the power cable 1102 can also include
a solid structural support 1114 disposed therein and filling the
inner lumen 1107. The outer return conductor 1106 and the
structural support 1114 can have a gas supply inlet 1116 extending
therethrough that is in fluid communication with the gas feed line
1112. The gas supply inlet 1116 can be, for example, a
radially-oriented tube that passes through the outer return
conductor 1106 and the solid structural support 1114.
[0076] Cryogenic gas that enters the gas supply inlet 1116 can be
carried forward toward the proximal end of the antenna 1104 within
the gas feed line 1112. Upon reaching the proximal end of the
antenna 1104, the cryogenic gas can be allowed to expand by passing
through a gas expansion conduit 1118 into an annular-shape gas
return conduit 1120 that is disposed around an outer circumference
of the inner lumen 1107. The cryogenic gas can provide additional
cooling as it expands into the larger annular gas return conduit
1120 due to the Joule-Thompson effect. The expanded gas can then be
carried back toward the proximal end of the cable 1102 to be
expelled through a gas vent 1122.
[0077] In addition to the various gas supply lines, the inner lumen
1107 can also house fluid supply lines configured to carry fluid
from a source connected to the proximal end of the cable 1102 to
the antenna 1104. For example, in some embodiments a fluid supply
line 1124 can extend from a proximal end of the cable 1102 through
the structural support 1114. After passing through the structural
support 1114, the fluid supply line can, in some embodiments,
expand to fill the annular conduit 1126 that extends along the
distal portion of the cable 1102 and the proximal portion of the
antenna 1104 between the outer wall of the gas feed line 1112 and
the inner wall of the gas return conduit 1120. Once the flowing
fluid reaches the distal end of the antenna 1104, it can be
injected into tissue surrounding the antenna via at least one
opening, e.g., a plurality of radially oriented openings 1128
formed in the antenna 1104 (similar to the operation of the antenna
200 described above). The introduction of fluid into the tissue can
aid in delivering thermal energy deeper into the tissue than is
possible with microwave ablation alone.
[0078] The fluid, typically saline, is in good thermal
communication with the returning cooled gas in the gas return
conduit 1120. As a result, the fluid can be cooled as it travels
along the length of the cable 1102 and antenna 1104. Indeed, in
some embodiments, the fluid can be cooled to below body temperature
prior to being injected into tissue. In the event that this is not
desirable, one or more heating elements can be placed within the
annular conduit 1126 to heat the fluid flowing therethrough. In the
illustrated embodiment, for example, two or more RF electrodes 1130
can be oriented along a longitudinal axis 1132 of the fluid annular
conduit 1126 for fluid supply conduit 1124. The electrodes 1130 can
be passed through the structural support 1114 to reach a power
supply at a proximal end of the cable 1102 and, if more than two
are used, emitting electrodes can be alternated with return
electrodes, as shown by emitter electrodes 1130A and return
electrodes 1130B in FIGS. 12 and 13.
[0079] The one or more heating elements disposed within the inner
lumen 1107 can be used to heat the fluid flowing therethrough to
any desired temperature. A temperature sensor (not shown) can be
positioned in the antenna 1104 proximal to the openings 1128 to
detect the temperature of the fluid being injected into the tissue
surrounding the antenna 1104. This feedback can be used to control
the amount of energy applied to the heating elements and achieve a
desired temperature for the fluid upon injection into tissue.
[0080] Furthermore, as a result of the fact that the fluid can be
cooled significantly by the cryogenic gas, any desired temperature
between the solid phase change temperature and the gas phase change
temperature (e.g., approximately 0.degree. C. and 100.degree. C.,
respectively, for saline) can be achieved by balancing the amount
of cooling gas provided and the RF energy applied to the one or
more electrodes 1130. This provides a novel benefit over prior art
solutions, as it has not previously been possible to introduce
fluid into the tissue at a temperature below at least room
temperature, if not body temperature.
[0081] FIG. 12 shows a cross-sectional view of the cable 1102 at
location A-A shown in FIG. 11. Visible in the figure are the outer
return conductor 1106, gas feed line 1112, solid structural support
1114, gas supply inlet 1116, fluid supply line 1124, RF emitter
electrodes 1130A, and RF return electrodes 1130B (extending in
alternating pairs around the circumference of the inner lumen
1107).
[0082] FIG. 13 similarly shows a cross-sectional view of the
antenna 1104 at location B-B shown in FIG. 11. Visible in this
figure are the outer return conductor 1106, gas feed line 1112, gas
expansion conduit 1118, annular gas return conduit 1120, annular
conduit 1126, RF emitter electrodes 1130A, and RF return electrodes
1130B (again shown extending in alternating pairs around the
circumference of the inner lumen 1107).
[0083] Those skilled in the art will appreciate that a number of
variations are possible on the embodiment shown in FIGS. 11-13. For
example, a number of alternative heating elements can be employed
in place of the RF electrodes, such as laser-based heaters,
resistive heaters, etc. Furthermore, there can be more than one
heating element disposed along the length of the assembly 1100. For
example, one or more proximal heating elements can be positioned
near a proximal end of the cable 1102 and used to control the
temperature of a fluid prior to entering a second, more distal set
of one of more heating elements. This second set of heating
elements can, for example, be positioned just proximal to the
plurality of openings 1128 and used to control the temperature of
the fluid prior to injection into tissue surrounding the antenna
1104. In this manner, the temperature of the microwave cable and
antenna proximal to the therapy zone can be controlled by the
first, proximal set of heating elements to be at a sub-therapeutic
temperature (e.g., typically 41.degree. C. or cooler to avoid
destroying tissue) and a second, more distal set of heating
elements can be used to further raise the temperature to a
therapeutic level (e.g., above 41.degree. C.).
[0084] Accordingly, methods according to the teachings of the
present invention can include, for example, delivering therapeutic
energy through a microwave antenna configured to be positioned
within a volume of tissue, while simultaneously delivering heated
fluid into the volume of tissue through at least one outlet port
formed in the microwave antenna. In certain embodiments, the fluid
can be heated solely by contacting the microwave antenna and
drawing excess heat therefrom. In other embodiments, however, the
excess heat from the microwave antenna may not heat the fluid to a
desired therapeutic temperature. In such an embodiment, one or more
heating elements disposed within the microwave antenna (e.g.,
within a fluid passageway formed in the antenna) can be used to
raise the temperature of the heated fluid prior to its introduction
into tissue surrounding the antenna.
[0085] In other embodiments, a method for ablating tissue according
to the teachings of the present invention can include delivering
therapeutic energy into a volume of tissue using a microwave
antenna, and cooling the microwave antenna by delivering a
cryogenic gas through at least one gas channel formed in the
microwave antenna. The method can further include delivering a
fluid into the volume of tissue through at least one fluid channel
formed in the microwave antenna, and controlling the temperature of
the fluid delivered into the volume of tissue using the cooling of
the cryogenic gas and one or more heating elements disposed within
the at least one fluid channel. As mentioned above, by balancing
the cooling effects of the cryogenic gas with the output of the one
or more heating elements, the fluid can be controlled to enter the
tissue surrounding the antenna at any temperature between, e.g., a
freezing and a boiling temperature of the fluid.
[0086] Any devices disclosed herein can be designed to be disposed
after a single use, or they can be designed for multiple uses. In
either case, however, the devices can be reconditioned for reuse
after at least one use. Reconditioning can include any combination
of the steps of disassembly of a device, followed by cleaning or
replacement of particular pieces, and subsequent reassembly. In
particular, a device can be disassembled, and any number of the
particular pieces or parts of the device can be selectively
replaced or removed in any combination. Upon cleaning and/or
replacement of particular parts, a device can be reassembled for
subsequent use either at a reconditioning facility or by a surgical
team immediately prior to a surgical procedure. Those skilled in
the art will appreciate that the reconditioning of a device can
utilize a variety of techniques for disassembly,
cleaning/replacement, and reassembly. Use of such techniques, and
the resulting reconditioned device, are all within the scope of the
present invention.
[0087] Preferably, any devices described herein will be processed
before surgery. First, a new or used instrument can be obtained
and, if necessary, cleaned. The instrument can then be sterilized.
In one sterilization technique, the instrument is placed in a
closed and sealed container, such as a plastic or TYVEK bag. The
container and its contents can then be placed in a field of
radiation that can penetrate the container, such as gamma
radiation, x-rays, or high-energy electrons. The radiation can kill
bacteria on the instrument and in the container. The sterilized
instrument can then be stored in the sterile container. The sealed
container can keep the instrument sterile until it is opened in the
medical facility. Other sterilization techniques can include beta
radiation, ethylene oxide, steam, and a liquid bath (e.g., cold
soak). In certain embodiments, the materials selected for use in
forming certain components may not be able to withstand certain
forms of sterilization, such as gamma radiation. In such a case,
suitable alternative forms of sterilization can be used, such as
ethylene oxide.
[0088] All papers and publications cited herein are hereby
incorporated by reference in their entirety. One skilled in the art
will appreciate further features and advantages of the invention
based on the above-described embodiments. Accordingly, the
invention is not to be limited by what has been particularly shown
and described, except as indicated by the appended claims.
* * * * *